Composite laminated materials, such as Fibre Reinforced Plastics (FRPs) often demonstrate significantly lower tensile strength when loaded perpendicular to the laminations, i.e. in the through-thickness direction, than when loaded in the plane of the laminations. It is therefore desirable to obtain accurate mechanical properties for the tensile strength of such laminated materials when loaded in the through-thickness direction.
A first known test method to determine the through-thickness tensile strength of laminated materials is the “curved beam” approach. In this method a load is applied to an angled or C-section coupon (test element) and the resulting through-thickness tensile strength is calculated from elasticity equations for a curved beam segment with cylindrical anisotropy. For laminated FRPs the results obtained through this method are only valid if the reinforcing fibres run continuously along the coupon. Disadvantages of this method include a significant sensitivity to flaws and variations in the test material, the results may be influenced by interaction between interlaminar shear stresses and through-thickness tensile stresses, giving a low apparent through-thickness tensile strength result, the test coupons must be manufactured to a dedicated L-shaped geometry, which means that in most cases the coupons cannot be extracted directly from candidate materials, and this method does not allow true through-thickness tensile strength data to be generated for materials containing reinforcement in more than one direction, such as FRPs containing woven fabric.
A second known test method is the “direct pull” approach, in which end blocks are bonded to opposite faces of a test coupon and pulled to give a direct through-thickness tensile strength reading. The coupons for this method can be extracted directly from any sufficiently thick laminate. Disadvantages of this method include the results are dependent on obtaining a good bond between the coupon and the end blocks and are very sensitive to alignment of the end blocks during bonding and the alignment of the grips of the apparatus used to apply the load (load frame). The bonding of the end blocks to some materials, for example thermoplastics, can be difficult and can result in premature coupon failure.